Difference between Pound-Force and Pound-Mass

In the SI (metric) system of units, kilograms are used for mass, and newtons for weight. There is no confusion as the units are different.

However, in the US system, pounds are used for both mass and weight. Confusing? You didn’t think it was going to be easy did you?

To differentiate between the two, I have developed the following table.

Property

Mass

Weight

SI Unit

Kilograms (kg)

Newtons (N)
= kg·m/s2

US Unit

Pounds-Mass (lb·m)

Pounds-Force (lb·f)
= lbm·ft/s2

Formula

W = mg

Mass is a constant property of an object

Weight varies with the gravitational field

It’s not as complicated as you think. Every time you see the unit of pounds, it may or may not have the acceleration of gravity included in it: W = mg. Unfortunately, you simply have to know whether you are dealing in mass or weight. On the bright side, it’s usually fairly obvious (and the units have to work).

g = 9.81 m/s2 = 32.09 ft/s2.

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About Bernie Roseke, P.Eng., PMP

Bernie Roseke, P.Eng., PMP, is the president of Roseke Engineering. As a bridge engineer and project manager, he manages projects ranging from small, local bridges to multi-million dollar projects. He is also the technical brains behind ProjectEngineer, the online project management system for engineers. He is a licensed professional engineer, certified project manager, and six sigma black belt. He lives in Lethbridge, Alberta, Canada, with his wife and two kids.